Ion-selective electrodes are well known in the patent literature.
In general, said electrodes comprise a crystalline membrane in the form of a pressed pellet of a material sensitive to the ion the activity of which is to be determined, located at the lower end of the electrode housing. The inner surface of the membrane is connected to the electrochemical cell arrangement through a reference element made either of an internal electrolyte of known activity in which the metal reference electrode is immersed, or through an appropriate metal system. The outer surface of the membrane is contacted with the test solution. Between both surfaces of the membrane, a potential difference is created. This potential is measured by means of a high impedance voltmeter connected to the reference element and to the test solution. The potential difference such measured and the logarithm of the activity of the ions to detect are linked by a linear relationship, the well known Nernst equation.
Numerous membranes comprised of metal sulfides, in particular Ag.sub.2 S membranes are disclosed in the art.
U.S. Pat. No. 3,591,464 discloses a crystalline membrane comprised of an intimate mixture of Ag.sub.2 S and another compound selected according to the nature of the ion the activity of which is to be determined. For example, to detect Cu.sup.+2 ions, mole percentage ratios of cupric sulfide to silver sulfide are in the range of 95% CuS/5% Ag.sub.2 S to 1% CuS/99% Ag.sub.2 S. Thus, CuS, PbS, CdS and AgSCN are associated to Ag.sub.2 S to detect Cu.sup.++, Pb.sup.++, Cd.sup.++ and SCN.sup.-, respectively.
U.S. Pat. No. 4,400,243 discloses a membrane comprising a mixture of at least four metal sulfides, each of them corresponding to a metal ion the activity of which is to be determined. For example, the mixture consists of from 15 to 65% by weight of silver sulfide, 15 to 65% by weight of cadmium sulfide, 30 to 65% by weight of lead sulfide and 3 to 9% of copper sulfide. The mixture can be obtained by coprecipitation, for example by adding to a sodium sulfide solution a mixed solution containing 0.3 mol cadmium nitrate, 0.3 mol lead nitrate, 0.3 mol silver nitrate and 0.1 mol copper nitrate. The membrane allows the detection of Ag.sup.+, Cd.sup.++, Pb.sup.++, Cu.sup.++ ions.
U.S. Pat. No. 3,563,874 discloses an imporous membrane comprised of a mixture of silver sulfide and at least 5 mol percent silver halide, for example silver chloride to detect Cl.sup.- ; the preferred mixtures permit a variation between 10 mol percent AgCl to 90 mol percent AgCl. In the case of mixed iodide sulfide and bromide sulfide membranes, the minimum halide content should be above 5 mol percent.
U.S. Pat. No. 4,096,049 discloses a membrane made of silver sulfide and which is superficially converted into a hardly soluble substance which has one ion in common with the ion to be detected, for example AgCl, AgBr or AgI to detect Cl.sup.-, Br.sup.-, I.sup.-, respectively.
U.S. Pat. No. 4,172,778 discloses a Ag.sub.3 SBr or Ag.sub.3 SI membrane to detect Ag.sup.+, I.sup.- or Br.sup.-. For example, Ag.sub.3 SI is obtained by precipitating equimolar quantities of sodium sulfide and sodium iodide with of a slight excess of silver nitrate.
U.S. Pat. No. 3,672,692 discloses a Ag.sub.2 S membrane for detecting sulfide ions in solution. Said patent teaches that the silver sulfide membrane must be of a relatively high state of purity since the presence of foreign ions or molecules, such as AgO, tends to cause porosity and poor mechanical properties.
U.S. Pat. No. 3,809,636 teaches to employ silver sulfide and silver selenide and/or telluride in combination in forming a silver ion sensing electrode, to detect silver ions. For example the membrane contains 10 to 30 wt percent of silver sulfide and 70 to 90 wt percent of silver telluride.
U.S. Pat. No. 4,549,953 discloses the combination of silver sulfide, silver selenide, silver telluride, and mercury sulfide, for example Ag.sub.4 SSe or Ag.sub.4 STe obtained by coprecipitation to form a membrane for measuring the activity of one of the three elements which compose the membrane.
As a summary, the Ag.sub.2 S crystalline membranes of the prior art comprise a mixture of silver sulfide and of a sulfide of another metal to detect silver or said other metal, or a mixture of silver sulfide and halide salt to detect said halide.